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MnO2 modified glass-ceramics
Journal of Non-Crystalline Solids 513 (2019) 64–69
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of TiO2 and thermal processing on morphological, structural and magnetic properties of Fe2O3/MnO2 modified glass-ceramics Satwinder Singh Danewaliaa, Supreet Kaurb, Neetu Bansalb, Savidh Khanb, K. Singhb, a b
T
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Division of Research and Development, Lovely Professional University, Phagwara 144411, India School of Physics and Materials Science, Thapar Institute of Engineering & Technology, Patiala 147004, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass-ceramics Bioactive Nucleation Superparamagnetism Hyperthermia Cancer
The present report is an investigation of morphological, structural and magnetic properties of multicomponent glass-ceramics with respect to content of nucleating agent (TiO2) as well as thermal processing parameters. Structural features of these glass-ceramics were explored by their crystalline phase analysis and morphological studies after sintering at various temperatures (850–1000 °C) for 2 h (h). The samples exhibited near superparamagnetic nature which would be helpful for heat generation purposes (such as in hyperthermia treatment of cancer). Magnetic properties of the glass-ceramics are dependent on the crystallized phases grown upon sintering. Concentration dependent role of TiO2 is discussed based upon the obtained results.
1. Introduction
sintering parameters. Nucleating agents such as ZrO2, Cr2O3, and TiO2 are added to the glass compositions to facilitate crystallization [14–17]. These additives play important role in the modification of properties of the glasses. Besides being good nucleating agent, TiO2 also improves bioactivity of the glasses [2,18–20]. It enhances wettability and thus bioactivity of the glass surfaces [21]. As an intermediate oxide, TiO2 may act as a network former or a network modifier depending on its relative concentration in glass batch. Thus, its tendency to interact with other components in different ways makes it worthwhile to investigate and understand its role during crystallization of glasses. Moreover, the present compositions exhibit two network formers (SiO2, P2O5), two network modifiers (Na2O, CaO) and two intermediate oxides (MnO2 and Fe2O3) along with TiO2 as a nucleating agent as well as a sintering aid. In this composition, SiO2-CaO-Na2O-P2O5 system is chosen as a bioactive matrix [12]. Fe2O3 and MnO2 has been added to introduce magnetic properties to the glasses with better bioactivity [12]. Some research groups reported the effect of TiO2 on structural and bioactive nature of the glasses [15–22]. However, its simultaneous effect on material characteristics at different temperatures has not been widely studied. Present work is the investigation of the effects of additive content (TiO2) and sintering temperature on the properties of bioactive glass-ceramics having magnetic elements. Previously, we reported the role of TiO2 in intriguing modification of magnetic properties of the glass-ceramics [2]. In the present work, the concentration of nucleating agent (TiO2) is extended up to 10 wt% and its effect was explored on properties of glass-ceramics obtained at various temperatures. This
Understanding of crystallization of the glasses is necessary to develop materials for specific applications. Properties of glass-ceramics are usually sensitive to the type and volume fraction of crystalline phase(s) grown in the glass-matrix using controlled heat-treatment [1,2]. For instance, in many cases formation of crystals within bioactive glass-ceramics retards its dissolution [3]. While for glasses designed for nuclear wastes management, the formation of certain crystals may have adverse effect on their chemical durability [4,5]. Crystallinity also influences optical properties such as optical sensing, lighting and scintillating characteristics of the glasses. Cao et al. reported that formation of fluoride nano-crystals improved the optical temperature sensing of the aluminosilicate glasses [6]. Controlled crystallization can be used to produce glass-ceramics with high optical sensitivity and efficient upconversion luminescence [7,8]. The relative ratio of magnetic/nonmagnetic crystalline phases in magnetic bioactive glass-ceramics determines their applicability in magnetically induced hyperthermia (MIH) treatment of cancer [9–12]. Volume fraction of the crystalline phases grown within a glass matrix depends upon many factors such as initial constituents of glasses, nucleating agents, sintering temperature and duration of sintering etc. Sintering is done basically to densify the powder compacts and is also meant for improving quality of crystals by their growth. Glasses with narrow sintering window usually crystallize upon sintering [13]. The properties of materials can be tuned to desired level by precise knowledge of their sintering properties and optimizing
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Corresponding author. E-mail address: [email protected] (K. Singh).
https://doi.org/10.1016/j.jnoncrysol.2019.03.013 Received 5 January 2019; Received in revised form 10 March 2019; Accepted 11 March 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
Journal of Non-Crystalline Solids 513 (2019) 64–69
S.S. Danewalia, et al.
work offers better understanding of the crystallization process of such glass-ceramics through their morphological, structural and magnetic characteristics.
20
% volume reduction
2. Materials and methods
TiO2 -free sample T5 T1 T7 T2 T10 T3
15
Glasses with compositions (45SiO2-25CaO-10Na2O-5P2O55Fe2O3-10MnO2) + TiO2 were prepared via conventional melt-quench technique. The details of the synthesis process are given elsewhere [2]. Glasses containing 1.25, 2.50, 3.75, 5.00, 7.50 and 10.00 extra wt% of TiO2 were labelled as T1, T2, T3, T5, T7 and T10, respectively. The quenched samples were crushed and ground in an agate mortar to obtain fine powders. These powdered samples were pelletized with the help of hydraulic press by applying pressure of 1.24 × 106 kN/m2 for 5 min each. The pellets were sintered at different temperatures i.e. 850 °C, 900 °C, 950 °C and 1000 °C with heating rate 5 °C min−1 for 2 h. Dimensions of the pellets were measured using Vernier callipers (least count = 0.01 mm). Change in the volume of these pellets as a consequence of sintering was calculated as follows:
%volume reduction =
Vi − Vf Vi
× 100
10
5
0
850
900
950
1000
Temperature (°C) Fig. 1. Shrinkage of the glass compacts after sintering at various temperatures.
(1) which rapidly increases the viscosity. In such a case, viscous flow is retarded and so does its contribution towards densification [27,29]. The densification process of present glass-ceramic pellets is a function of crystallization at higher temperatures. These results are similar to that of Fu et al.; they observed monotonic decrease in the viscosity of TiO2 free glasses with temperature. TiO2 contained glasses exhibited a composition dependent variation in viscosity because of crystallization and crystal growth process [30]. In the present glass-ceramics, higher percentage of TiO2 (T7 and T10) acts as an effective sintering aid since both these glass-ceramics showed maximum volume change as shown in Fig. 1.
Vi and Vf represent volume of the pellets before and after sintering. Morphology of the fractured surface of the glass-ceramic pellets was investigated by scanning electron microscopy (SEM) (JEOL/EO, version 1.0). For structural analysis, these glass-ceramics were subjected to powder X-ray diffraction (XRD) with CuKα radiations (λ = 1.54 Å). The samples were scanned in 2θ range 10–90° at a scanning rate ~3° min−1 with step size of 0.0170°. Magnetic hysteresis curves of the crystallized glass-ceramic pellets were plotted on Lakeshore VSM-7404 vibrating sample magnetometer (VSM) with an application of magnetic field ± 10 kOe at room temperature. For spectroscopic studies, 1 mg of the sample was dissolved in diluted HCl solution (HCl:H2O = 1:9) and sonicated for 10 min. Supernatant of solution was used for further studies to avoid any undissolved powder. Absorption spectra of the samples in UV–visible region (200–800 nm) were taken on HITACHI U-3900H UV–visible spectrophotometer.
3.2. Morphological analysis The SEM images of the samples (Figs. 2 and 3) provide direct evidence of the morphological development of these glass-ceramics upon heating. Fig. 2 (a-d) depicts the effect of sintering temperature on the representative sample (T5). For the glass-ceramic obtained at 850 °C, powder particles of wide size distribution can be seen on the surface. The population of smaller particles reduced at 900 °C, while much denser microstructure was obtained at 950 °C and 1000 °C. The microstructure of the sample at higher temperatures is not significantly different, which supports the volume shrinkage data discussed above. The material undergoes considerable morphological changes during sintering and the process completes mainly through three steps. First stage represents particles that lack physical integrity and pellets are less dense. Neighbouring particles begin to form necks at contact points in the second stage of densification [31]. Finally, the pores are filled and material reaches maximum densification. Moreover, if heating temperature of the glass pellets falls in their crystallization region then sintering and crystallization occur simultaneously [32]. Higher magnification micrographs (Fig. 3) show the aforementioned necking process where smaller particles started merging into bigger particles. The formation of tiny crystals as a consequence of heating can also be seen embedded within the glassy matrix which is further discussed in the next section. SEM images were also taken for glass-ceramics (different TiO2 concentrations) obtained at 950 °C i.e. the temperature at which maximum densification was observed (Fig. 2 (e, f)). T10 exhibits bigger particles on its surface (as compared to T1, Fig. 2 (e)) and dense morphology which does not differ significantly than that of T5. In contrast, T1 seems to be less dense than TiO2 rich samples (T5 and T10). It shows that higher amount of TiO2 may be helpful for the better densification of these glass-ceramics.
3. Results and discussion 3.1. Densification Generally, the glass pellets shrink during sintering due to densification process [23,24]. It can be observed that TiO2–free sample exhibits lowest reduction in volume compared to TiO2–rich samples (Fig. 1). It indicates that TiO2 results in better sintering of the pellets. TiO2 in glass-ceramics decreases the viscosity of the parent glass at high temperatures [25], which results in better densification of the specimen. Similar results showing higher densification in glass systems with respect to TiO2 content has also been observed by Mukherjee et al. [25]. Densification of present pellets exhibited more dependence on composition up to 950 °C. Thereafter, the compositional dependence tended to cease and all the pellets experienced more or less similar volume contraction. Thus, 950 °C seems to be best sintering temperature for the present glass-ceramics. The volume reduction is maximum at this temperature for the samples except T5 and T1. At low TiO2 concentrations, the densification 0continued up to 1000 °C. However, TiO2 rich samples experienced maximum densification at 950 °C. It appears that TiO2 may be added to glasses for improved sinterability at lower temperatures. Lower densification at 1000 °C may be due to loss of some of the low melting point components/species (such as Na2O) from the glasses [26]. Secondly, crystallization of the glasses also affects the densification process. Mass transport in vitreous materials occurs via viscous flow [27,28]. Once the glass transition temperature (Tg) is attained, the glass starts flowing and facilitates the densification. At higher temperatures, surface crystallization may start 65
Journal of Non-Crystalline Solids 513 (2019) 64–69
S.S. Danewalia, et al.
Fig. 2. Scanning electron micrographs showing morphology of (a) T5 after sintering at 850 °C (b) 900 °C (c) 950 °C (d) 1000 °C (e) T1 after sintering at 950 °C (f) T10 after sintering at 950 °C.
reported that phase separation, concentration of nucleating agents and sintering schedule have a drastic effect on the crystallization process [35]. As a result, identity and/or volume fraction of the crystalline phases grown inside the glassy network varies, which eventually determines the structure sensitive properties of the material. In present glass-ceramics, iron titanium oxide is important phase as per their magnetic behaviour is concerned. Partial substitution of iron ion (Fe3+) by titanium ions (Ti3+/4+) in the Fe2O3 lattice structure alters the magnetic behaviour of the glass-ceramics. It has been observed that the maximum magnetization of the glass-ceramics (discussed in next section) is proportional to the intensity of the diffraction peaks of iron titanium oxide phase. The crystallite size of this phase was calculated from its highest intensity peak using Scherrer formula
3.3. Structural analysis Fig. 4 shows the XRD patterns of the glass-ceramics after sintering at 950 °C. The phases grown upon sintering are iron titanium oxide ((Fe2.5Ti0.5)1.04O4, ICDD card No.-00-051-1587), calcium silicate (Ca3Si2O7, ICDD card No.-00-023-0124), calcium manganese silicate (CaMn14SiO24, ICDD card No.-00041-1368) and sodium aluminium silicate (Na6.8Al6.3Si9.7O32, ICDD card No. - 01-079-0994). Small amount of alumina was anticipated to become part of the compositions during melting of the batches. Similar observations have also been made by other researchers for glasses prepared in alumina crucibles [33]. Alumina present in amounts < 5 mol% acts as glass-former while higher amount is reported to act as network modifier in the glasses [34]. It is 66
Journal of Non-Crystalline Solids 513 (2019) 64–69
S.S. Danewalia, et al.
[36,37]. The crystallite size of this phase is below 100 nm for the present glass-ceramics at 950 °C. Fig. 5 represents the XRD patterns of T5 glass-ceramic at various temperatures. At lower temperatures, iron titanium oxide, iron oxide and sodium aluminium silicate are major crystalline phases. At higher temperatures, calcium ions started diffusing into the lattice and formed calcium silicate. Peaks corresponding to sodium aluminium silicate are found to be broadened with temperature which signifies reduction in crystallite size of this phase. It supports the dissociation of this phase and subsequent transformation into other phases. Evolution of crystalline phases can also be assessed from the relative increase in the intensities of the diffraction peaks at the cost of other crystalline phases. Peaks at ~22, 28 and 33° corresponding to calcium silicate and iron titanium oxide phases become more intense with respect to concentration of TiO2 and higher sintering temperature. Diffraction peak at ~35° splits into many neighbouring peaks, owing to calcium silicate phase. Amidst these phase transformations, the degree of crystallinity varied with minimum (~19%) at 850 °C and maximum (~25%) at 900 °C, while sample exhibited nearly the same degree of crystallinity (~23%) at 950 °C and 1000 °C. With increase in temperature, the crystallite size of iron titanium oxide was also observed to increase usually due to higher crystal growth at higher temperatures [38].
Fig. 3. SEM image of T5 showing necking of the particles during densification of the pellet.
3.4. Magnetic properties The magnetic hysteresis curves of the glass-ceramics obtained at 950 °C are shown in Fig. 6. With low coercive field below 100 Oe and non-linear behaviour, these curves indicate near superparamagnetic nature of these glass-ceramics. Amorphous systems generally consist of Fe3+ ions surrounded by distorted octahedron of O2−ions. Aperiodic lattice in glassy systems leads to randomly oriented symmetry axes. Any deviation from the average coordination causes local ordering of a cluster of ions [39]. Such local ordering along with the growth of nanometric magnetic crystals (as discussed in the XRD section) are responsible for the near superparamagnetic behaviour of these glassceramics. Superparamagnetic materials are able to generate heat under alternating magnetic field due to Neel's relaxation [40]. Thus, the magnetic nature of these glass-ceramics indicates their effectiveness as thermo-seeds in magnetically induced hyperthermia treatment of cancer [41,42]. However, the bioactive and the biocompatible nature of these samples must be determined prior to claim their use as biomaterials. TiO2 seems to take over the control of magnetic behaviour at concentrations > 2.5 wt% and serves to improve magnetic saturation
Fig. 4. XRD patterns of the glass-ceramics sintered at 950 °C. Symbols represent: Υ-Na6.8Al6.3Si9.7O32, o- Ca3Si2O7, %-(Fe2.5Ti0.5)1.04O4, Δ-CaMn14SiO24, ϕ-unidentified peak.
Fig. 5. XRD patterns of T5 showing evolution of crystalline phases at different temperatures. Symbols have same meaning as given in Fig. 4.
Fig. 6. Hysteresis curve of all compositions at 950 °C. Inset shows bar graph representing maximum magnetisation of the samples with respect to their compositions. 67
Journal of Non-Crystalline Solids 513 (2019) 64–69
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Fig. 7. Absorption spectra of the glass-ceramics obtained at 950 °C.
Fig. 8. Area under absorption band at 230 nm for the samples heat treated at 950 °C.
(Inset Fig. 6). Variation in saturation magnetization of these glassceramics is consistent with crystallization of magnetic phases as mentioned in previous section. Saturation magnetization of the glass-ceramics increased with increase in TiO2 content with T1 and T10 as exceptions. In the presence of TiO2, the nucleation followed by crystallization of the magnetic phases increases, which may account for the increase in saturation magnetization. At extreme concentration (T10), TiO2 might started taking part in glass formation leaving lesser fractions of titania available for nucleation purposes. That might have been a reason of decline in the magnetization of T10. Change in trend in properties of glasses with respect to concentration of intermediate oxides are reported to be ascribed to change in structural role of the intermediate oxides [43]. However, magnetization is not a function of crystallinity of magnetic phase(s) alone. It also depends upon the oxidation states of the magnetic ions (iron in present case). Transition metal ions usually absorb electromagnetic waves in UV–visible region and provide fruitful information about their oxidation states. UV–visible absorption spectra of these samples (Fig. 7) exhibit an intense absorption band centred at ~230 nm and a relatively smaller absorption band at ~330 nm. Absorption band at 330 nm arises due to transfer of electron from O2– ion to fill the empty d0 orbital of Ti4+ cation in [TiO4] 4− structural units [44]. Steele and Douglas also found absorption at these wavelengths for iron doped sodium silicate glasses [45]. Transition metal ions with partially filled d-orbitals invoke electronic transitions within the d-shells, called d-d transitions. Fe3+ (d5configuration) ions are reported to produce strong UV-absorption [44]. Absorption band at 230 nm is associated with Fe3+ ions with octahedral symmetry. The area under the curve obtained corresponding to the absorption band at 230 nm is maximum for T1 and T7 sample as can be seen in Fig. 8. Interaction of electromagnetic waves with a medium containing magnetic ions leads to magnetic-optic effects such as Faraday rotation. In general, along with large Faraday rotation large optical absorption is also observed [46]. This effect has contributions from both electric and magnetic dipole transitions. The latter contribution is proportional to the saturation magnetization [46]. The similarity between trend in absorption and saturation magnetization of these samples indicate that the abnormal magnetic saturation of T1 and T7 samples is related to abundance of Fe3+ ions compared to its other oxidation states. General increase in magnetization and coercivity of the samples was observed at higher temperatures, which attributes to the coarsening of the crystals and enhanced degree of crystallinity [47]. Accompanying this, the hysteresis curves tend to saturate at lower applied magnetic fields. It would be beneficiary as it enables the samples to produce more
Fig. 9. Effect of temperature and TiO2 content on Mr/Ms ratio.
heat at clinically possible magnetic fields [40]. The effect of TiO2 content on hysteresis curves can be understood more easily by comparing Mr/Msvalues for the samples at different temperatures (Fig. 9). The Mr/Ms ratio close to unity indicates better magnetic saturation at low magnetic fields. At lower temperature (850 °C), a random variation is seen in Mr/Msratio with respect to TiO2 content. However, on achieving effective sintering (950 °C), TiO2 controls the curve shape as indicated by improved Mr/Ms ratio towards unity.
4. Conclusion TiO2 can be added to the glass compositions to achieve better sintering at relatively lower temperatures. At higher concentrations (7.5 wt% in present case), TiO2 seems to changes its function from network modifier to network former. The crystallization of iron titanium oxide phase determines the magnetic behaviour of these glassceramics. There was good correlation between magnetization and electromagnetic absorption due to Fe3+ ions. TiO2 may help in regulating the magnetic behaviour of the glass-ceramics by tuning their hysteresis curves. These glass-ceramics are nearly superparamagnetic which makes them attractive materials for hyperthermia treatment of cancer. 68
Journal of Non-Crystalline Solids 513 (2019) 64–69
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Acknowledgements
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of TiO2 and thermal processing on morphological, structural and magnetic properties of Fe2O3/MnO2 modified glass-ceramics Satwinder Singh Danewaliaa, Supreet Kaurb, Neetu Bansalb, Savidh Khanb, K. Singhb, a b
T
⁎
Division of Research and Development, Lovely Professional University, Phagwara 144411, India School of Physics and Materials Science, Thapar Institute of Engineering & Technology, Patiala 147004, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Glass-ceramics Bioactive Nucleation Superparamagnetism Hyperthermia Cancer
The present report is an investigation of morphological, structural and magnetic properties of multicomponent glass-ceramics with respect to content of nucleating agent (TiO2) as well as thermal processing parameters. Structural features of these glass-ceramics were explored by their crystalline phase analysis and morphological studies after sintering at various temperatures (850–1000 °C) for 2 h (h). The samples exhibited near superparamagnetic nature which would be helpful for heat generation purposes (such as in hyperthermia treatment of cancer). Magnetic properties of the glass-ceramics are dependent on the crystallized phases grown upon sintering. Concentration dependent role of TiO2 is discussed based upon the obtained results.
1. Introduction
sintering parameters. Nucleating agents such as ZrO2, Cr2O3, and TiO2 are added to the glass compositions to facilitate crystallization [14–17]. These additives play important role in the modification of properties of the glasses. Besides being good nucleating agent, TiO2 also improves bioactivity of the glasses [2,18–20]. It enhances wettability and thus bioactivity of the glass surfaces [21]. As an intermediate oxide, TiO2 may act as a network former or a network modifier depending on its relative concentration in glass batch. Thus, its tendency to interact with other components in different ways makes it worthwhile to investigate and understand its role during crystallization of glasses. Moreover, the present compositions exhibit two network formers (SiO2, P2O5), two network modifiers (Na2O, CaO) and two intermediate oxides (MnO2 and Fe2O3) along with TiO2 as a nucleating agent as well as a sintering aid. In this composition, SiO2-CaO-Na2O-P2O5 system is chosen as a bioactive matrix [12]. Fe2O3 and MnO2 has been added to introduce magnetic properties to the glasses with better bioactivity [12]. Some research groups reported the effect of TiO2 on structural and bioactive nature of the glasses [15–22]. However, its simultaneous effect on material characteristics at different temperatures has not been widely studied. Present work is the investigation of the effects of additive content (TiO2) and sintering temperature on the properties of bioactive glass-ceramics having magnetic elements. Previously, we reported the role of TiO2 in intriguing modification of magnetic properties of the glass-ceramics [2]. In the present work, the concentration of nucleating agent (TiO2) is extended up to 10 wt% and its effect was explored on properties of glass-ceramics obtained at various temperatures. This
Understanding of crystallization of the glasses is necessary to develop materials for specific applications. Properties of glass-ceramics are usually sensitive to the type and volume fraction of crystalline phase(s) grown in the glass-matrix using controlled heat-treatment [1,2]. For instance, in many cases formation of crystals within bioactive glass-ceramics retards its dissolution [3]. While for glasses designed for nuclear wastes management, the formation of certain crystals may have adverse effect on their chemical durability [4,5]. Crystallinity also influences optical properties such as optical sensing, lighting and scintillating characteristics of the glasses. Cao et al. reported that formation of fluoride nano-crystals improved the optical temperature sensing of the aluminosilicate glasses [6]. Controlled crystallization can be used to produce glass-ceramics with high optical sensitivity and efficient upconversion luminescence [7,8]. The relative ratio of magnetic/nonmagnetic crystalline phases in magnetic bioactive glass-ceramics determines their applicability in magnetically induced hyperthermia (MIH) treatment of cancer [9–12]. Volume fraction of the crystalline phases grown within a glass matrix depends upon many factors such as initial constituents of glasses, nucleating agents, sintering temperature and duration of sintering etc. Sintering is done basically to densify the powder compacts and is also meant for improving quality of crystals by their growth. Glasses with narrow sintering window usually crystallize upon sintering [13]. The properties of materials can be tuned to desired level by precise knowledge of their sintering properties and optimizing
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Corresponding author. E-mail address: [email protected] (K. Singh).
https://doi.org/10.1016/j.jnoncrysol.2019.03.013 Received 5 January 2019; Received in revised form 10 March 2019; Accepted 11 March 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
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work offers better understanding of the crystallization process of such glass-ceramics through their morphological, structural and magnetic characteristics.
20
% volume reduction
2. Materials and methods
TiO2 -free sample T5 T1 T7 T2 T10 T3
15
Glasses with compositions (45SiO2-25CaO-10Na2O-5P2O55Fe2O3-10MnO2) + TiO2 were prepared via conventional melt-quench technique. The details of the synthesis process are given elsewhere [2]. Glasses containing 1.25, 2.50, 3.75, 5.00, 7.50 and 10.00 extra wt% of TiO2 were labelled as T1, T2, T3, T5, T7 and T10, respectively. The quenched samples were crushed and ground in an agate mortar to obtain fine powders. These powdered samples were pelletized with the help of hydraulic press by applying pressure of 1.24 × 106 kN/m2 for 5 min each. The pellets were sintered at different temperatures i.e. 850 °C, 900 °C, 950 °C and 1000 °C with heating rate 5 °C min−1 for 2 h. Dimensions of the pellets were measured using Vernier callipers (least count = 0.01 mm). Change in the volume of these pellets as a consequence of sintering was calculated as follows:
%volume reduction =
Vi − Vf Vi
× 100
10
5
0
850
900
950
1000
Temperature (°C) Fig. 1. Shrinkage of the glass compacts after sintering at various temperatures.
(1) which rapidly increases the viscosity. In such a case, viscous flow is retarded and so does its contribution towards densification [27,29]. The densification process of present glass-ceramic pellets is a function of crystallization at higher temperatures. These results are similar to that of Fu et al.; they observed monotonic decrease in the viscosity of TiO2 free glasses with temperature. TiO2 contained glasses exhibited a composition dependent variation in viscosity because of crystallization and crystal growth process [30]. In the present glass-ceramics, higher percentage of TiO2 (T7 and T10) acts as an effective sintering aid since both these glass-ceramics showed maximum volume change as shown in Fig. 1.
Vi and Vf represent volume of the pellets before and after sintering. Morphology of the fractured surface of the glass-ceramic pellets was investigated by scanning electron microscopy (SEM) (JEOL/EO, version 1.0). For structural analysis, these glass-ceramics were subjected to powder X-ray diffraction (XRD) with CuKα radiations (λ = 1.54 Å). The samples were scanned in 2θ range 10–90° at a scanning rate ~3° min−1 with step size of 0.0170°. Magnetic hysteresis curves of the crystallized glass-ceramic pellets were plotted on Lakeshore VSM-7404 vibrating sample magnetometer (VSM) with an application of magnetic field ± 10 kOe at room temperature. For spectroscopic studies, 1 mg of the sample was dissolved in diluted HCl solution (HCl:H2O = 1:9) and sonicated for 10 min. Supernatant of solution was used for further studies to avoid any undissolved powder. Absorption spectra of the samples in UV–visible region (200–800 nm) were taken on HITACHI U-3900H UV–visible spectrophotometer.
3.2. Morphological analysis The SEM images of the samples (Figs. 2 and 3) provide direct evidence of the morphological development of these glass-ceramics upon heating. Fig. 2 (a-d) depicts the effect of sintering temperature on the representative sample (T5). For the glass-ceramic obtained at 850 °C, powder particles of wide size distribution can be seen on the surface. The population of smaller particles reduced at 900 °C, while much denser microstructure was obtained at 950 °C and 1000 °C. The microstructure of the sample at higher temperatures is not significantly different, which supports the volume shrinkage data discussed above. The material undergoes considerable morphological changes during sintering and the process completes mainly through three steps. First stage represents particles that lack physical integrity and pellets are less dense. Neighbouring particles begin to form necks at contact points in the second stage of densification [31]. Finally, the pores are filled and material reaches maximum densification. Moreover, if heating temperature of the glass pellets falls in their crystallization region then sintering and crystallization occur simultaneously [32]. Higher magnification micrographs (Fig. 3) show the aforementioned necking process where smaller particles started merging into bigger particles. The formation of tiny crystals as a consequence of heating can also be seen embedded within the glassy matrix which is further discussed in the next section. SEM images were also taken for glass-ceramics (different TiO2 concentrations) obtained at 950 °C i.e. the temperature at which maximum densification was observed (Fig. 2 (e, f)). T10 exhibits bigger particles on its surface (as compared to T1, Fig. 2 (e)) and dense morphology which does not differ significantly than that of T5. In contrast, T1 seems to be less dense than TiO2 rich samples (T5 and T10). It shows that higher amount of TiO2 may be helpful for the better densification of these glass-ceramics.
3. Results and discussion 3.1. Densification Generally, the glass pellets shrink during sintering due to densification process [23,24]. It can be observed that TiO2–free sample exhibits lowest reduction in volume compared to TiO2–rich samples (Fig. 1). It indicates that TiO2 results in better sintering of the pellets. TiO2 in glass-ceramics decreases the viscosity of the parent glass at high temperatures [25], which results in better densification of the specimen. Similar results showing higher densification in glass systems with respect to TiO2 content has also been observed by Mukherjee et al. [25]. Densification of present pellets exhibited more dependence on composition up to 950 °C. Thereafter, the compositional dependence tended to cease and all the pellets experienced more or less similar volume contraction. Thus, 950 °C seems to be best sintering temperature for the present glass-ceramics. The volume reduction is maximum at this temperature for the samples except T5 and T1. At low TiO2 concentrations, the densification 0continued up to 1000 °C. However, TiO2 rich samples experienced maximum densification at 950 °C. It appears that TiO2 may be added to glasses for improved sinterability at lower temperatures. Lower densification at 1000 °C may be due to loss of some of the low melting point components/species (such as Na2O) from the glasses [26]. Secondly, crystallization of the glasses also affects the densification process. Mass transport in vitreous materials occurs via viscous flow [27,28]. Once the glass transition temperature (Tg) is attained, the glass starts flowing and facilitates the densification. At higher temperatures, surface crystallization may start 65
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Fig. 2. Scanning electron micrographs showing morphology of (a) T5 after sintering at 850 °C (b) 900 °C (c) 950 °C (d) 1000 °C (e) T1 after sintering at 950 °C (f) T10 after sintering at 950 °C.
reported that phase separation, concentration of nucleating agents and sintering schedule have a drastic effect on the crystallization process [35]. As a result, identity and/or volume fraction of the crystalline phases grown inside the glassy network varies, which eventually determines the structure sensitive properties of the material. In present glass-ceramics, iron titanium oxide is important phase as per their magnetic behaviour is concerned. Partial substitution of iron ion (Fe3+) by titanium ions (Ti3+/4+) in the Fe2O3 lattice structure alters the magnetic behaviour of the glass-ceramics. It has been observed that the maximum magnetization of the glass-ceramics (discussed in next section) is proportional to the intensity of the diffraction peaks of iron titanium oxide phase. The crystallite size of this phase was calculated from its highest intensity peak using Scherrer formula
3.3. Structural analysis Fig. 4 shows the XRD patterns of the glass-ceramics after sintering at 950 °C. The phases grown upon sintering are iron titanium oxide ((Fe2.5Ti0.5)1.04O4, ICDD card No.-00-051-1587), calcium silicate (Ca3Si2O7, ICDD card No.-00-023-0124), calcium manganese silicate (CaMn14SiO24, ICDD card No.-00041-1368) and sodium aluminium silicate (Na6.8Al6.3Si9.7O32, ICDD card No. - 01-079-0994). Small amount of alumina was anticipated to become part of the compositions during melting of the batches. Similar observations have also been made by other researchers for glasses prepared in alumina crucibles [33]. Alumina present in amounts < 5 mol% acts as glass-former while higher amount is reported to act as network modifier in the glasses [34]. It is 66
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[36,37]. The crystallite size of this phase is below 100 nm for the present glass-ceramics at 950 °C. Fig. 5 represents the XRD patterns of T5 glass-ceramic at various temperatures. At lower temperatures, iron titanium oxide, iron oxide and sodium aluminium silicate are major crystalline phases. At higher temperatures, calcium ions started diffusing into the lattice and formed calcium silicate. Peaks corresponding to sodium aluminium silicate are found to be broadened with temperature which signifies reduction in crystallite size of this phase. It supports the dissociation of this phase and subsequent transformation into other phases. Evolution of crystalline phases can also be assessed from the relative increase in the intensities of the diffraction peaks at the cost of other crystalline phases. Peaks at ~22, 28 and 33° corresponding to calcium silicate and iron titanium oxide phases become more intense with respect to concentration of TiO2 and higher sintering temperature. Diffraction peak at ~35° splits into many neighbouring peaks, owing to calcium silicate phase. Amidst these phase transformations, the degree of crystallinity varied with minimum (~19%) at 850 °C and maximum (~25%) at 900 °C, while sample exhibited nearly the same degree of crystallinity (~23%) at 950 °C and 1000 °C. With increase in temperature, the crystallite size of iron titanium oxide was also observed to increase usually due to higher crystal growth at higher temperatures [38].
Fig. 3. SEM image of T5 showing necking of the particles during densification of the pellet.
3.4. Magnetic properties The magnetic hysteresis curves of the glass-ceramics obtained at 950 °C are shown in Fig. 6. With low coercive field below 100 Oe and non-linear behaviour, these curves indicate near superparamagnetic nature of these glass-ceramics. Amorphous systems generally consist of Fe3+ ions surrounded by distorted octahedron of O2−ions. Aperiodic lattice in glassy systems leads to randomly oriented symmetry axes. Any deviation from the average coordination causes local ordering of a cluster of ions [39]. Such local ordering along with the growth of nanometric magnetic crystals (as discussed in the XRD section) are responsible for the near superparamagnetic behaviour of these glassceramics. Superparamagnetic materials are able to generate heat under alternating magnetic field due to Neel's relaxation [40]. Thus, the magnetic nature of these glass-ceramics indicates their effectiveness as thermo-seeds in magnetically induced hyperthermia treatment of cancer [41,42]. However, the bioactive and the biocompatible nature of these samples must be determined prior to claim their use as biomaterials. TiO2 seems to take over the control of magnetic behaviour at concentrations > 2.5 wt% and serves to improve magnetic saturation
Fig. 4. XRD patterns of the glass-ceramics sintered at 950 °C. Symbols represent: Υ-Na6.8Al6.3Si9.7O32, o- Ca3Si2O7, %-(Fe2.5Ti0.5)1.04O4, Δ-CaMn14SiO24, ϕ-unidentified peak.
Fig. 5. XRD patterns of T5 showing evolution of crystalline phases at different temperatures. Symbols have same meaning as given in Fig. 4.
Fig. 6. Hysteresis curve of all compositions at 950 °C. Inset shows bar graph representing maximum magnetisation of the samples with respect to their compositions. 67
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Fig. 7. Absorption spectra of the glass-ceramics obtained at 950 °C.
Fig. 8. Area under absorption band at 230 nm for the samples heat treated at 950 °C.
(Inset Fig. 6). Variation in saturation magnetization of these glassceramics is consistent with crystallization of magnetic phases as mentioned in previous section. Saturation magnetization of the glass-ceramics increased with increase in TiO2 content with T1 and T10 as exceptions. In the presence of TiO2, the nucleation followed by crystallization of the magnetic phases increases, which may account for the increase in saturation magnetization. At extreme concentration (T10), TiO2 might started taking part in glass formation leaving lesser fractions of titania available for nucleation purposes. That might have been a reason of decline in the magnetization of T10. Change in trend in properties of glasses with respect to concentration of intermediate oxides are reported to be ascribed to change in structural role of the intermediate oxides [43]. However, magnetization is not a function of crystallinity of magnetic phase(s) alone. It also depends upon the oxidation states of the magnetic ions (iron in present case). Transition metal ions usually absorb electromagnetic waves in UV–visible region and provide fruitful information about their oxidation states. UV–visible absorption spectra of these samples (Fig. 7) exhibit an intense absorption band centred at ~230 nm and a relatively smaller absorption band at ~330 nm. Absorption band at 330 nm arises due to transfer of electron from O2– ion to fill the empty d0 orbital of Ti4+ cation in [TiO4] 4− structural units [44]. Steele and Douglas also found absorption at these wavelengths for iron doped sodium silicate glasses [45]. Transition metal ions with partially filled d-orbitals invoke electronic transitions within the d-shells, called d-d transitions. Fe3+ (d5configuration) ions are reported to produce strong UV-absorption [44]. Absorption band at 230 nm is associated with Fe3+ ions with octahedral symmetry. The area under the curve obtained corresponding to the absorption band at 230 nm is maximum for T1 and T7 sample as can be seen in Fig. 8. Interaction of electromagnetic waves with a medium containing magnetic ions leads to magnetic-optic effects such as Faraday rotation. In general, along with large Faraday rotation large optical absorption is also observed [46]. This effect has contributions from both electric and magnetic dipole transitions. The latter contribution is proportional to the saturation magnetization [46]. The similarity between trend in absorption and saturation magnetization of these samples indicate that the abnormal magnetic saturation of T1 and T7 samples is related to abundance of Fe3+ ions compared to its other oxidation states. General increase in magnetization and coercivity of the samples was observed at higher temperatures, which attributes to the coarsening of the crystals and enhanced degree of crystallinity [47]. Accompanying this, the hysteresis curves tend to saturate at lower applied magnetic fields. It would be beneficiary as it enables the samples to produce more
Fig. 9. Effect of temperature and TiO2 content on Mr/Ms ratio.
heat at clinically possible magnetic fields [40]. The effect of TiO2 content on hysteresis curves can be understood more easily by comparing Mr/Msvalues for the samples at different temperatures (Fig. 9). The Mr/Ms ratio close to unity indicates better magnetic saturation at low magnetic fields. At lower temperature (850 °C), a random variation is seen in Mr/Msratio with respect to TiO2 content. However, on achieving effective sintering (950 °C), TiO2 controls the curve shape as indicated by improved Mr/Ms ratio towards unity.
4. Conclusion TiO2 can be added to the glass compositions to achieve better sintering at relatively lower temperatures. At higher concentrations (7.5 wt% in present case), TiO2 seems to changes its function from network modifier to network former. The crystallization of iron titanium oxide phase determines the magnetic behaviour of these glassceramics. There was good correlation between magnetization and electromagnetic absorption due to Fe3+ ions. TiO2 may help in regulating the magnetic behaviour of the glass-ceramics by tuning their hysteresis curves. These glass-ceramics are nearly superparamagnetic which makes them attractive materials for hyperthermia treatment of cancer. 68
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Acknowledgements
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